Paper of specialized performance characteristics may be created by applying a thin layer of coating material to one or both sides of the paper. The coating is typically a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder which may be of the organic type or of a synthetic composition. In addition, rosin, gelatins, glues, starches or waxes may be applied to paper for sizing.
Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers and other applications requiring specialized paper qualities.
Various devices have been employed in the past to apply coatings to paper, either directly, or by first applying it to a roll, as in a size press. Once coating has been applied to the substrate, it is necessary to meter the coating to a desired thickness and uniform level. Uneven coating thickness will produce blemishes and quality variances in the finished paper, and is highly undesirable.
One approach to metering the coating is to position a flexible blade against the backing roll downstream of the coating pond. Blades are low in cost, easy to replace, and offer a certain degree of flexibility, yet a simple blade directly engages the coating along an edge, and, in some applications, blade defects may be transferred to the coating, causing corresponding defects in the paper. Often a fiber will be caught on the blade and cause a nonuniform coating or stripe. In addition, when high machine speed or a high solids pigmented coating is used, an uncontrollable coating film with a nonuniform, time-dependent pattern is normally generated by the blade on the substrate.
In both web coaters and in metering size presses, a rotating rod placed within a rod holder is an effective alternative to the use of a flexible blade. In this approach, because of the speeds at which the rod is required to rotate, water is often circulated in the rod holder to clean, lubricate, and cool the rod for easy rotation. Thus, while the rotating rod alleviates the problem of paper defects that result from the flexible blade, operating problems and quality defects result if water is allowed to get into the coating. In an attempt to prevent this from happening, rod holders utilizing a press fit of the rod to the rod holder housing have been developed. However, a tight fit places heavy demands on the motor that rotates the rod, and the resultant friction that is generated between the rod and the rod housing causes wear on the rod metering device, necessitating more frequent replacement.
To reduce the costs in lost production when a papermaking machine is repaired, the down time of the coating apparatus that results from having to service or replace the metering applicator should be minimized.
What is needed is a metering device that combines the flexibility, low cost, and ease of replacement of the flexible blade with the metering performance of the rotating rod.